Set g_texture_cache to NULL upon shutdown. Fixes a crash introduced in previous commits.

Source/Plugins/Plugin_VideoDX9/Src/main.cpp

@@ -212,6 +212,7 @@ void VideoBackend::Shutdown()
 		delete g_texture_cache;
 		delete g_renderer;
 		g_renderer = NULL;
+		g_texture_cache = NULL;
 	}
 	D3D::Shutdown();
 }